WORK AND

POWER!

What isWork? Work is done when a force

motion of an objectcauses a change in

Motion of object must be in the SAME direction

as the force. Work = ZERO if thereis no motion.

An Example

A weightlifter lifts abarbell over his head. Is work being done? Yes, the motion

and the force are in the

same direction.

Once thehis head,

barbell is overhe holds itthere

for3 seconds. Is work being

done? No, there is no motion.

Another Example

Dora picks up her backback to put it

on. After it is on, she

runs outthe door to start

exploring. Is work being done on

thebag? Yes, when the bag is

being lifted. Not while Dora is

carryingthe bag out the door.

Helpful

Hint

This is a GENERAL

but… If I use the words:

guide and not a set rule,

Push, Pull, Lift, or Throw, WORK was done if

motion occurred If you are walking or running, you are working Carry or Hold, NO WORK was done

because any motion is in a different

direction from the force Exception: Carrying an object

up stairs is work

The

Math

WorkW

==

Force

F

x Distancex d

Work ismeasured

in:Joules

Newton * Meters 1 kg*1 Joule = 1 N*m = * m

What isa Joule?

1 Joule = 1 N * m

1 joule

of energy is

the energyneeded

tolift an

apple from yourwaist

toyour head.

PracticeImagine a fatherplaying with hisdaughter by lifting

herrepeatedly in the air.How much work doeshe do on the child witheach lift, assuming helifts her 2.0m andexerts a force of 190N?

Power

(HONORS ONLY)

Power is

the

rate atwhich work

isdone

Power =

work/timeSI Unit

1 wattpower

is the Wattis the amountneeded to

do

of1

J of work in 1 s

Work vs.

Power

Running and walking up a flight ofstairs requires the same amount ofwork – How?Same force, same distance = same

work

Running does it faster – What doestell you about Power?

that

Power

Practice

While rowing across the lake during a race,

John does 3,960 joules of work on the oars in60s.

What is the rower’s power output?

Power

Practice

It takes 100 kJ of work

to lift an elevator 18m. If this is done in 20s, what is the average power of the elevator during the process?(hint: convert to joules)

Power

Practice

If a person pushes a

lawn mower with a force of 500N over distance of 20m in120s, what was thepower required?

a

ENERGY!

Energy!

Energy is the capacity to do

work,and ability to

causemotion

SI Unit= Joules (J) 1

1

1

Joule

Joule

Joule

=

=

=

1 N * 1 m

(1 kg *1 ) * 1m

1 kg *

Energy!

There are 7 forms of energy But there are 2 broad terms that we use:kinetic and

potential.Each of the 7 typeshave a form of kinetic and

potential.Basically:Potenti

alKinetic =

We will look at

PE a bit more.

= stored energyenergy of motionthe 7, then zoom in on KE

and

The 7

Types

of Energy!

Mechanical Energy1)Chemical EnergyElectrical Energy Nuclear Energy Thermal Energy Radiant EnergySound Energy

2)

3)

4)5)

6)

7)

And remember, each one of these 7 has a form ofpotential energy and each has a form of kinetic

energy

The 7

Types

of Energy!

Mechanical Energy – the energy stored in an object

1) due to its motion or position.

Mech Kinetic Energy: a moving car, a cheetah runningMech Potential Energy: stored in a spring, compressed ball,or a plane in the air

Examples: A running kid, a flying plane, a potted plant sitting on a ledge

The 7

Types

of Energy!

2) Chemical Energy – Energy stored in the

bonds that holdthem together.

Chem Potential Energy: stored in a battery or ethanol

This energy changes into another form during chemical reactions. Examples:

Food, batteries, humans, gasoline, trees, etc.

The 7

Types

of Energy!

3) Electrical Energy – Energy of electronsmoving around a circuit.

Elec Kinetic Energy: movement of electronsElec Potential Energy: stored in a capacitor

4) Nuclear Energy – Energy associated with

the nucleusElec Kinetic Energy: movement of

electronsElec Potential Energy: stored in the forces binding thenucleus together; unstable nuclei

The 7

Types

of Energy!

5) Thermal Energy – type of energy that

comes from

heatand vibration of

molecules.6) Light/Radiant Energy – energy associated withelectromagnetic spectrum and light

waves.Radiant Kinetic Energy: movement of photons7) Sound Energy – Energy that is associated

withthemovement of sound waves.

Sound Kinetic Energy: movement of atoms/molecules

Potential Energy Energy stored due to position or shape.

Ithas the ability or “potential” to move.

There are 2 types: 1) Elastic – energy stored by things that

stretch (or compress). Examples?

- Rubber bands, sling shots, elastic,muscles, a spring, a ball bouncing,

etc…

Potential Energy Remember energy stored due to position. It has theability or “potential” to move. There are 2 types:

2) Gravitational Potential Energy- energy stored byobjects that are lifted off the

ground. Grav Potent Energy = mass * gravity * heightA 65kg rock climber ascends a cliff. What is the climber’sGPE at a point 35m above the base of the cliff?

GPEGPE GPE

===

m * g * h65kg * 9.8 * 35m22295 joules

Kinetic Energy

Kinetic – energy of motion.Amount of energy depends

Object's mass and velocity. Formula

on

v2KE = ½ m *Units = Joules

Kinetic Energy Remember kinetic energy

is the energy of

motion. Formula

KE = ½ m *

v2

Units = Joules (remember 1J = 1N

* 1

Practice: What is the Kinetic Energy

of a44kg

cheetah

running at

31m/s?

v2 KE KE KE KE

====

½½½

m ** 31 *916m2/s2

(44 kg)(44 kg)

21,142 joules

Law of Conservation of

Energy

Energy can never be created and never

destroyed. Energy can only change from one to another.

form

Net Energy is the total amount of energy in a system. The total or net amount of energy must stay the same

when energy changes forms.

Law of Conservation

of Energy

Before the drop If the 4kg soccer ball is held at a height of

5m, how much GPE does it have? GPE = M * G * H = 196 joules of GPE

If the 4kg soccer ball is held at a height of

5m, how much KE does it have? KE = ½ mv2 = 0 joules of KE (not moving)

Law of Conservation of

Energy

After the drop… If the 4kg soccer ball is dropped, how much

GPEdoes it have when it strikes the ground at 0m? GPE = M * G * H = 0 joules of GPE (height is 0m) If the 4kg soccer ball is dropped, how much

GPEdoes it have when it strikes the ground at 0m? KE = ½ mv2 = 196 joules of KE

M = 4kg V = 9.899 m/s at impact

Law of Conservation

of Energy

Recap: So for an object that is going to simply “fall”

GPE at the Drop Point = KE at the Impact Point Because all of the GPE is converted to KE throughout

fall

the

This only works for “falling” objects though…. A roller coaster is not falling, per se…

But a

Roller Coaster is

not just a

fall…1stAs we start at the top of

thehill…

Energy Changes on the

Roller Coaster

We saw on previous slide that the GPE the car has at the 1sttop of

thethe hill.

hill will turn into KE as the car travels down

1stAnd that the KE the car has at the bottom of the

hillwillturn into GPE as the

But is this a perfect

car travels

up the

next

hill.

transformationenergy from GPE to KE and back?Nope. Energy is

of

lost

as heat and sound.

Energy

Conversions

Explain what

energy we are starting

with and whatenergy

weare ending with for the

following

products: A Lightbulb

Start with Electricity_

End with Light Waste Heat

Can you determine the other 4 energy

Remember: Law of Conservation of

conversions?Energy…A

A A A

fanbattery fire glowstick

StartStart Start Start

withwith with with

EndEnd End End

withwith with with

Waste

Waste

WasteWaste

Energy Conversion Example: As a swing moves back and forth, the energy

converting from one form to another…..

Why will the swing eventually stop?

is

In what form will that energy be transferred into?

THERMAL ENERGY & SPECIFIC HEAT

Measuring Heat

You have probably used a thermometer to find the temperature outside.

Temperature is the measurement we use to quantify the sensations of hot and cold.

Measuring Heat Temperature is a measure of average

kinetic energy of the molecules of an object.

As temperature increases, so does the average kinetic energy of the molecules.

Measuring Heat It is difficult to

measure the speed of individual molecules in an object since they are much too small to see.

We commonly use indirect measurement to find an object’s temperature.

The expansion of liquid in a thermometer is directly proportional to the increase in temperature.

Temperature Scales Fahrenheit scale was

developed in 1714 by Gabriel Fahrenheit(1686-1936), a German physicist.

In 1742, Anders Celsius (1701-44), a Swedish astronomer, invented a temperature scale in which there were 100 degrees between freezing and boiling.

Thermal Energy and Heat Thermal energy is the

sum of all the kinetic energy of the molecules of a material.

If two containers of soup are at the same temperature, a large pot contains more thermal energy because it takes more energy to heat it.

Measuring HeatHeat is the flow of

thermal energy due to a temperature difference between two objects.

In the scientific sense, heat occurs only when there is a difference in temperature.

Flow of Heat If you double the

mass of the object you are going to heat, you need twice as much energy to increase the temperature.

Flow of Heat Scottish chemist Joseph Black

(1728-99) developed the theory of specific heat in 1760.

Heat added to boiling water causes water to turn to gas, but it does not raise the temperature.

Black called the heat used to boil or melt substances latent heat because it could not be sensed with a thermometer.

Latent means "hidden".

Specific Heat Specific heat is a property of a substance. The specific heat of a substance is the

amount of heat needed to raise the temperature of one gram by one degree Celsius.

One unit used to measure heat is the calorie.

Specific Heat of Various Metals

Specific Heat of Water Water has a high specific heat. Water has greater resistance to temperature

change than does air.

1st Law of Thermodynamics

In an isolated system, the total amount of thermal energy remains constant.

When hot water and ice are placed in a closed system:

1. The energy lost by the hot water is equal to

2. The energy gained by the ice.

3. Eventually, the contents reach thermal equilibrium.

Heat TransferThermal energy flows from

a material at a higher temperature to a material at a lower temperature.

This general process is called heat transfer.

Conduction and convection both require matter for the energy to transfer.

Conduction and InsulatorsMaterials that

conduct heat easily are called thermal conductors.

Those that conduct heat poorly are called thermal insulators.

ConvectionConvection is the

transfer of heat by the actual motion of a fluid (liquid or gas) in the form of currents.

Convection comes from a Latin word meaning "to carry together."

Convection in gasesThrough the process of

convection, the air surrounding your body warms up, rises, and carries the heat away.

Convection in liquids The hot water at the

bottom of the pot rises to the top of the pot, and is replaced by the cooler water.

Next the cooler water is heated.

If this did not happen, we would have to rely on the slower method of conduction to boil a pot of water.

RadiationRadiation is the

direct transfer of energy by electromagnetic waves.

When electromagnetic waves from the sun strike Earth, some are absorbed and others are reflected.

RadiationA white sand beach is a poor

absorber and poor emitter of radiation.

A black road surface is a good absorber and good emitter of radiation.